JPH0528023B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a contact type image sensor, and more particularly to a contact type image sensor that improves the output signal characteristics of the contact type image sensor.

[Prior art]

A contact image sensor is composed of a plurality of light receiving elements, ie, a light receiving element array, and a circuit for switching and scanning the element array. The length of this light-receiving element array is the same as the width of the document, and the document is read by placing it in close contact with the document or by one-to-one imaging through an optical system such as an optical fiber array or lens array. This allows the imaging optical path length to be shorter than that of a MOS image sensor or a CCD image sensor, making it possible to significantly downsize a device equipped with the contact image sensor.

FIG. 1 is a plan view showing the characteristic parts of a conventional contact type image sensor, and FIG.
FIG. 3 is a cross-sectional view taken along line -A'. As shown in the figure, the contact image sensor 10 has a plurality of divided electrodes T1, T2,..., Tn formed on an insulating substrate 11 at appropriate intervals, and is overlapped with a part of the divided electrodes T1 to Tn to conduct photoconductivity. It has a laminated structure in which a transparent conductive thin film 12 is formed, and a transparent conductive thin film 13 is further formed on this thin film 12. In addition, the divided electrodes T1 to
Bonding wires W1, W2,
..., Wn to the switching circuit 14.

As the insulating substrate 11, glass, ceramic,
A metal plate with an insulated surface or a silicon wafer is used as the divided electrodes T1 to Tn.
Mo, W, Ta or Ni, etc., and as the photoconductive thin film 12, hydrogenated amorphous silicon,
Se-As-Te, CdS, CdSe, etc. are used, and as the transparent conductive thin film 13, _SnO2 , _{In2O3 ,} etc. are used, respectively. In addition, Figures 1 and 2
In the figure, the area where the lower divided electrodes T1 to Tn and the upper transparent conductive thin film 13 intersect is the part that functions as the light receiving elements L1, L2,...Ln, and the entire photoconductive thin film 12 receives light. It does not act as an element.

Next, FIG. 3 is a circuit diagram of the contact type image sensor 10 shown in FIGS. 1 and 2. This close-contact image sensor 10 includes photodiodes PD1, PD2,..., PDn and a capacitor PC, respectively.
When an original image is formed on the light receiving elements L1, L2, ..., Ln equivalently represented by a parallel circuit of 1, PC2, ..., PCn, the photodiode PD1
A photocurrent according to the light intensity flows through PD2 to PD2, and the stray capacitance of the divided electrodes T1 to Tn and the MOS
Signal charges corresponding to the magnitude of the photocurrent are accumulated in signal charge storage capacitors C1, C2, . Next, when the shift register 15 turns on the MOS transistors S1, S2, ..., Sn sequentially for a certain period of time, that is, performs switching scanning, the signal charge accumulated in each capacitor C1 to Cn is transferred to the signal output line 16. The current flowing through the load resistor 17 at this time is extracted as a signal corresponding to the image information of the document. Note that the power source 18 is a bias power source for each of the light receiving elements L1 to Ln.

By the way, the conventional contact type image sensor 10 is
Since the MOS transistors S1 to Sn are switched and scanned and the discharge of the signal charge accumulated in each signal charge storage capacitor C1 to Cn is output, each
Spike noise generated when MOS transistors S1 to Sn are turned on is transmitted from the gate to signal output line 1.
6 may be mixed in as noise charges.
For example, if the amount of signal charge accumulated in capacitor C1 is 1
[pC], the gate voltage of MOS transistor S1 is 5
[V], and the junction capacitance between the gate and source is 1 [pF], the signal output line 16 due to spike noise.
The amount of noise charge mixed in is 1 [pF] x 5 [V] = 5
[pC], which becomes larger than the signal charge. Further, the time for discharging the signal charges in each of the capacitors C1 to Cn is only 1 [μsec] or less, and the current flowing through the resistor 17 due to the discharge is weak. Conventionally, this required a noise cancel circuit and a high-speed, high-gain amplifier, which posed a problem in that it was not possible to downsize the device equipped with the contact image sensor 10.

Furthermore, since the divided electrodes T1 to Tn are arranged in parallel on the substrate 1 at high density, the line capacitance between adjacent divided electrodes is of a size that cannot be ignored.
For example, the width of each divided electrode T1 to Tn is 70 [μm],
The length is 2 [cm], and the interval between parallel electrodes is 125 [μm].
In this case, the line capacitance is 1 to 3 [pF].
FIG. 4 is a circuit diagram of a contact type image sensor that takes such line capacitance into consideration. As shown in Figure 4
When the MOS transistor S1 is turned on and the signal charge in the signal charge storage capacitor C1 is discharged, the other capacitors C2 and C3 are discharged via the line capacitance C', causing crosstalk in the signal charges. There was a problem.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and provides a signal that is large enough to correspond to the image information of a document, without the need for external circuits such as a noise canceling circuit and an amplifier circuit, and without generating noise charges. It is an object of the present invention to provide a contact type image sensor which can obtain the following characteristics and which does not cause crosstalk due to line capacitance of divided electrodes.

[Structure of the invention]

In order to achieve the above object, according to the present invention,
A plurality of divided electrodes are provided on a substrate, a photoconductor and a transparent conductor are sequentially laminated on this to form a plurality of light-receiving elements, and the charges accumulated in the plurality of light-receiving elements by light irradiation are used as a signal. In a contact image sensor that outputs output via an output line, the plurality of light-receiving elements are divided into a plurality of light-receiving element groups, and charges accumulated in each light-receiving element are discharged by turning on corresponding to each light-receiving element group. It consists of a plurality of first switching circuits each discharging, and a high input impedance voltage follower circuit constructed of complementary MOS transistors, and a plurality of first switching circuits each outputting a voltage corresponding to the charge accumulated in each light receiving element. A buffer amplifier, and a plurality of second MOS transistors, each of which is made up of a complementary MOS transistor whose gate is supplied with a drive signal with a different polarity, and which are sequentially turned on by the drive signal to sequentially take out the outputs of the plurality of buffer amplifiers.
It consists of a switching circuit and a complementary MOS transistor whose gates are supplied with drive signals of different polarities, and the outputs of the second switching circuit are connected in common and are turned on when the second switching circuit is driven. and a third switch circuit that outputs the output of the second switching circuit to the signal output line.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 5 is a circuit diagram of a contact type image sensor according to the present invention. Note that in FIG. 5, parts that perform the same functions as those in FIG. 3 are designated by the same reference numerals. Further, the contact type image sensor according to the present invention has a plurality of blocks 20,
Since each block has the same structure, only block 20 will be explained.

Signal charge storage capacitor C1, C that stores signal charge
2,...,Cl is discharged with the signal charge, and the light receiving element L
A first switching circuit S11, S12, .
In addition, photodiodes PD1, PD2,
..., PDl and the light receiving elements L1, L represented by a parallel circuit of capacitors PC1, PC2, ..., PCl
2,..., Ll, one of which is commonly wired and connected to the negative side of the power supply 18, and the other is connected to the second switching circuit S21, S22,..., via the amplifier A1, A2,..., Al, respectively. Connected to S2l. Furthermore, each of the light receiving elements L1 to Ll is connected to the signal output line 16 via a third switching circuit S3. The amplifiers A1 to Al are voltage follower circuits having a circuit as shown in FIG. 6, and have high input impedance and low output impedance. Therefore, the amplifiers A1 to Al output a voltage approximately equal to the voltage applied to the non-inverting input terminal +, that is, the signal charges in the signal charge storage capacitors C1 to Cl can be held without being discharged.

First, second, and third switching circuits S
11 to S1l, S21 to S2l, and S3 are controlled to be turned on and off at appropriate timings by a shift register 15. Second and third switching circuits S21 to S2l and S
3 are CMOS transistors (complementary MOS
It is composed of a transistor) and an inverter, and prevents the generation of spike noise during on/off control of the switching circuit, and prevents noise charges from being mixed into the signal charges in the signal charge storage capacitors C1 to Cl. That is, the second and third switching circuits S21 to S2l and S3 are composed of n-channel MOS transistors and p-channel MOS transistors.
By connecting the MOS transistors, the signals applied to each gate from the control circuit 15 have opposite polarities, and the spike noises generated on the signal output line 16 when the gates are closed cancel each other out and become smaller. In addition, a third switching circuit S
3 is closed only when outputting the signal charges of the signal charge storage capacitors C1 to Cl connected to the switching circuit S3, and is kept open except when outputting the signal charges, thereby realizing high-speed output of the signal charges. It's getting old. In other words, when all the light receiving elements are connected in parallel to the signal output line 16 as in the conventional contact type image sensor, the load capacitance of the circuit becomes the sum of the junction capacitances of the second switching circuits S21 to S2n, and the signal charge output This will significantly reduce speed. For example, if the junction capacitance of the second switching circuit is several tens to several hundreds [pF], the load capacitance will be several thousand to several ten thousand [pF]. Therefore, as in the present invention, a third switching circuit S is provided between each of the plurality of light receiving elements and the signal output line 16.
By providing the switching circuit S3 and closing the switching circuit S3 only when outputting the signal charges of the light receiving elements L1 to Ll corresponding to the switching circuit S3, the load capacitance at least when outputting the signal charges is reduced, and the load capacitance is reduced to correspond to the signal charges. This enables high-speed output of voltage.

Next, to explain the operation of the contact type image sensor according to the present invention, first, the original image is transmitted to each light receiving element L.
When an image is formed on the photodiodes PD1 to Ll, a photocurrent corresponding to the light intensity flows through the photodiodes PD1 to PDl, and signal charges are accumulated in each of the signal charge storage capacitors C1 to Cl. At this time, the output voltage of each amplifier A1 to Al has a magnitude corresponding to the signal charges accumulated in the capacitors C1 to Cl. Next, the third switching circuit S3 is turned on, and the second switching circuits S21 to S2l are turned on in sequence to increase the output voltage of each amplifier A1 to Al, that is, a voltage corresponding to the signal charge of each capacitor C1 to Cl. is outputted via the signal output line 16. Furthermore, the second
The switching scan of the first switching circuits S11 to S1l is delayed by an appropriate time from the switching scan of the first switching circuits S21 to S2l, that is, when the switching element S23 is turned on, the switching scan of the first switching circuits S11 to S1l is started, and each signal charge storage capacitor C1
The signal charges of Cl to Cl are discharged, and the light receiving elements L1 to
Reset Ll.

Next, the SN of the contact type image sensor according to the present invention
Calculate the ratio. Assuming that the photocurrent flowing through the light receiving element L1 due to light irradiation is 1 [nA] and the accumulation time of the signal charge storage capacitor C1 is 1 [msec], then 1 [nA]×1 m seconds] = 1 [pC] signal charge is accumulated, and the output voltage of amplifier A1 is 1 [pC]/
1 [pF] = 1 [V]. On the other hand, assuming that the junction capacitance of the second switching circuit S21, that is, the difference in junction capacitance between the gate and source of each of the n-channel MOS transistor and the p-channel MOS transistor, is 0.1 [pF] and the gate voltage is 5 [V]. The amount of noise charge caused by spike noise is 0.1 [pF]
×5 [V] = 0.5 [pC], and if the capacitance of the output signal line 16 is 100 [pF], the amount of noise charge is 0.5
It corresponds to [pC]/100 [pF] = 0.005 [V]. Therefore, the SN ratio of the signal charge is 1/0.005=200, or 46 [dB]. Furthermore, when the signal charge output, that is, the output voltage of the amplifiers A1 to Al, is output, there is no charging or discharging of the signal charge storage capacitors C1 to Cl, so there is no charge transfer via the line capacitance between adjacent light receiving elements. . Therefore, no crosstalk occurs.

〔Effect of the invention〕

As explained above, according to the present invention, a buffer amplifier is provided for each light-receiving element, and this buffer amplifier is configured from a voltage follower circuit with high input impedance configured by complementary MOS transistors, and furthermore, the output of the buffer amplifier is A complementary type in which drive signals with different polarities are applied to the gates as the second and third switching circuits that take out the
Since it is configured using MOS transistors, 1. It is possible to prevent the generation of switching noise with a simple configuration and prevent noise charges from being mixed in with the signal charge. 2. There is no need for a noise cancel circuit, etc. This provides effects such as obtaining a good signal-to-noise ratio.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing the characteristic parts of a conventional contact type image sensor, and Fig. 2 is a plan view showing characteristic parts of a conventional contact type image sensor.
A cross-sectional view taken along line A', Figure 3 is a circuit diagram of a conventional contact type image sensor, Figure 4 is a circuit diagram of a contact type image sensor considering the line capacitance between divided electrodes, and Figure 5 6 is a circuit diagram of the contact type image sensor according to the present invention, and FIG. 6 is a circuit diagram of the amplifier shown in FIG. 5. 15...Shift register, 16...Signal output line, 18...Power supply, L1 to Ll...Light receiving element,
PD1 to PDl...Photodiode, PC1 to
PCl...Capacitor, C1 to Cl...Signal charge storage capacitor, A1 to Al...Amplifier, S11 to S
1l...first switching circuit, S21 to S
2l...second switching circuit, S3...third
switching circuit.

Claims (1)

[Claims] 1. A plurality of divided electrodes are provided on a substrate, a photoconductor and a transparent conductor are sequentially laminated thereon to form a plurality of light receiving elements, and the plurality of light receiving elements are formed by light irradiation. In a close-contact image sensor that outputs electric charge accumulated in a sensor via a signal output line, the plurality of light receiving elements are divided into a plurality of light receiving element groups, and each light receiving element is turned on in correspondence with each light receiving element group. It consists of a plurality of first switching circuits that discharge the charges accumulated in each element, and a voltage follower circuit with a high input impedance composed of complementary MOS transistors, which corresponds to the charges accumulated in each light receiving element. A plurality of buffer amplifiers, each of which outputs a voltage, and a plurality of complementary MOS transistors whose gates are supplied with drive signals of different polarities, and a plurality of second switching circuits that are sequentially turned on by the drive signals and sequentially take out the outputs of the plurality of buffer amplifiers. and a complementary MOS transistor whose gates are given drive signals with different polarities, the outputs of the second switching circuit are commonly connected, and are turned on when the second switching circuit is driven, and the second switching circuit is turned on when the second switching circuit is driven. and a third switch circuit that outputs the output of the switching circuit to the signal output line. 2. The contact type according to claim 1, wherein the plurality of first switching circuits are sequentially turned on with a predetermined time delay from the sequential turning-on operation of the plurality of second switching circuits. image sensor.